Lithographic projection exposure methods are predominantly used nowadays for producing semiconductor components and other finely structured components, such as, for example, masks for photolithography. In this case, use is made of masks (reticles) or other patterning devices that bear or form the pattern of a structure to be imaged, for example a line pattern of a layer of a semiconductor component. The pattern is positioned in a projection exposure apparatus between an illumination system and a projection lens in the region of the object plane of the projection lens and is illuminated with an illumination radiation shaped by the illumination system. The radiation modified by the pattern travels through the projection lens as projection radiation, the projection lens imaging the pattern with a reduced scale onto the substrate to be exposed. The surface of the substrate is arranged in the image plane of the projection lens optically conjugate to the object plane. The substrate is generally coated with a radiation-sensitive layer (resist, photoresist).
One of the aims in the development of projection exposure apparatuses is to produce lithographically structures having smaller and smaller dimensions on the substrate. In the case of semiconductor components, for example, smaller structures lead to higher integration densities, which generally has an expedient effect on the performance of the structured components produced. One approach involves working with shorter wavelengths of the electromagnetic radiation. By way of example, optical systems have been developed which use electromagnetic radiation from the extreme ultraviolet range (EUV) in particular having operating wavelengths in the range of between 5 nanometers (nm) and 30 nm, in particular of 13.5 nm.
An EUV projection exposure apparatus of the generic type including an illumination system of the generic type is known from U.S. Pat. No. 7,473,907 B2. The illumination system is designed for receiving EUV radiation of an EUV radiation source and for shaping illumination radiation from at least one portion of the received EUV radiation. The illumination radiation is directed into an illumination field in an exit plane of the illumination system during exposure operation, wherein the exit plane of the illumination system and the object plane of the projection lens advantageously coincide. The illumination radiation is characterized by specific illumination parameters and is incident on the pattern within the illumination field with a defined shape and size.
The EUV radiation source, which may be a plasma source, for example, is arranged in a source module separate from the illumination system, the source module generating a secondary radiation source at a source position in an entrance plane of the illumination system.
The image quality in EUV lithography is determined firstly by the projection lens, secondly also by the illumination system and the EUV radiation source and/or the source module. For a good image quality, a predefined spatial intensity distribution, which should normally be as uniform as possible, should be present for example within the illumination field. Furthermore, a predefined angular distribution should be present. A change in the distribution can be caused both in terms of power and in terms of position by a degradation of the EUV radiation source and/or an alteration of the alignment state of the secondary radiation source in relation to the illumination system.
For determining the alignment state of the secondary radiation source in relation to the illumination system or optical components of the illumination system, the illumination system includes an alignment state determining system, e.g. similar or identical to that in U.S. Pat. No. 7,473,907 B2. The alignment state determining system includes at least one alignment detector configured, during exposure operation, to receive a portion of the EUV radiation emerging from the secondary radiation source and to generate therefrom an alignment detector signal representative of the alignment state of the secondary radiation source.
Electromagnetic radiation from the EUV range generally cannot be focused or guided with the aid of refractive optical elements, since the short wavelengths are absorbed by the known optical materials that are transparent at longer wavelengths. Therefore, mirror systems are used in EUV lithography. The illumination system includes at least one mirror module including at least one used mirror element and at least one alignment mirror element. During exposure operation the at least one used mirror element contributes to the shaping of the illumination radiation incident on the illumination field and the at least one alignment mirror element reflects a portion of the EUV radiation of the secondary radiation source directly or indirectly in the direction of the alignment detector.
The plasma generated in the EUV radiation source includes ions and electrons that move at high velocity. As a result, despite possible protection measures, contamination of components of the illumination system can occur, e.g. of a mirror surface of a used mirror element. The contamination can adversely influence the image quality. In order to address this, for example, it is possible to carry out a cleaning of the contaminated components in the installed state, see e.g. U.S. Pat. No. 7,671,347 B2. Degradations based on other mechanisms are also possible.